英文:
Aerospike Query Return Highest Value
问题
我正在尝试为我的Aerospike数据库创建一个查询,该查询将返回特定bin中的最高值,类似于MySQL中MAX()函数的工作方式。例如,如果我有一个这样的集合:
+--------------+---------+
| filename | version |
+--------------+---------+
| alphabet.doc | 4 |
| people.doc | 2 |
| alphabet.doc | 6 |
| people.doc | 3 |
+--------------+---------+
我需要的是只返回具有最高版本号的文件名。目前,我可以添加一个类似这样的过滤器:
stmt := db.NewStatement(DBns, DBset, "filename", "version")
stmt.Addfilter(db.NewEqualFilter("filename", "alphabet.doc"))
// 运行数据库查询
records := runQuery(stmt)
有人知道如何做到这一点吗?
英文:
I'm trying to create a query for my Aerospike database, that would return the highest value in a specific bin; similar to the way that the MAX() function works in MySQL. For example, if I had a set like this:
+--------------+---------+
| filename | version |
+--------------+---------+
| alphabet.doc | 4 |
| people.doc | 2 |
| alphabet.doc | 6 |
| people.doc | 3 |
+--------------+---------+
What I need is to only return the filename with the highest version number. At the moment I can add a filter like this:
stmt := db.NewStatement(DBns, DBset, "filename", "version")
stmt.Addfilter(db.NewEqualFilter("filename", "alphabet.doc"))
// run database query
records := runQuery(stmt)
Anyone know how to do this?
答案1
得分: 4
您可以将Lua用户定义的函数(UDF)应用于查询,以高效地过滤结果。
例如,这是一个流式UDF示例,它将返回具有最大版本号的记录:
function maxVersion(stream, bin)
-- 流函数无法直接返回记录对象,因此我们首先需要映射到Map数据类型。
local function toArray(rec)
local result = map()
result['filename'] = rec['filename']
result['version'] = rec['version']
return result
end
local function findMax(a, b)
if a.version > b.version then
return a
else
return b
end
end
return stream : map(toArray) : reduce(findMax)
end
使用Go客户端,您可以像这样执行该函数:
stmt := NewStatement(ns, set)
recordset, _ := client.QueryAggregate(nil, stmt, "udfFilter", "maxVersion")
for rec := range recordset.Results() {
res := rec.Record.Bins["SUCCESS"].(map[interface{}]interface{})
fmt.Printf("filename with max. version: %s (ver. %d)\n", res["filename"], res["version"])
}
我已经将一个完整可工作的示例上传到了Gist上,链接在这里:https://gist.github.com/jhecking/b98783bea7564d610ea291b5ac47808c
您可以在这里找到有关如何使用流式UDF进行查询聚合的更多信息:http://www.aerospike.com/docs/guide/aggregation.html
英文:
You can apply a Lua user-defined function (UDF) to the query to filter the results efficiently.
E.g. here is a Stream UDF that would return the record with the max. version number:
function maxVersion(stream, bin)
-- The stream function cannot return record objects directly,
-- so we have to map to a Map data type first.
local function toArray(rec)
local result = map()
result['filename'] = rec['filename']
result['version'] = rec['version']
return result
end
local function findMax(a, b)
if a.version > b.version then
return a
else
return b
end
end
return stream : map(toArray) : reduce(findMax)
end
Using the Go client you would execute the function like this:
stmt := NewStatement(ns, set)
recordset, _ := client.QueryAggregate(nil, stmt, "udfFilter", "maxVersion")
for rec := range recordset.Results() {
res := rec.Record.Bins["SUCCESS"].(map[interface{}]interface{})
fmt.Printf("filename with max. version: %s (ver. %d)\n", res["filename"], res["version"])
}
I've uploaded a fully working example as a Gist here: https://gist.github.com/jhecking/b98783bea7564d610ea291b5ac47808c
You can find more information about how to work with Stream UDFs for query aggregation here: http://www.aerospike.com/docs/guide/aggregation.html
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